Examining neural circuits crucially relies on the ability to activate or silence individual circuit components to subsequently assess their impact on other parts of the circuit and their influence on behavior. Recent refinements of viral tools for gene delivery have allowed optogenetic methods to target cells based on specific cell types, localization, and connectivity. The physiological dissection of targeted circuits has been extremely successful in the mouse brain, but remains of limited use in non-human primate brain. We plan to develop and test a new generation of viral tools that will allow us to both activate and suppress different cell types in non-human primate models. To accomplish our aims we have assembled an expert team with complementary expertise composed of a biochemist and photobiologist (John Spudich), a molecular neuroscientist (Roger Janz), and a systems and computational neuroscientist (Valentin Dragoi). Our approach builds upon recently discovered anion-conducting channelrhodopsins (ACRs), which perform with perfect anion selectivity, photosensitivity orders of magnitude greater than current optogenetic rhodopsins, and enable highly efficient neuron hyperpolarization. We believe that our ACR constructs will open a new chapter in targeted neuro-suppression. In addition, we will use new neuron-activating (depolarizing) cation-conducting channelrhodopsins (CCRs) that have ~3-fold greater unitary conductance, faster recovery from excitation, and higher sodium selectivity than the commonly used channelrhodopsin-2. We will construct viral vectors encoding ACR-CCR pairs and, using spectrally different ACRs, ACR-ACR pairs, enabling efficient wavelength-selected neuron activation or suppression in large populations. The effectiveness of these viral vectors will be tested in cultured and in situ mouse neurons and in the primary visual cortex (V1) of behaving monkeys. Developing these powerful tools will be invaluable for probing neural circuits in non-human primate models, finally allowing the interrogation of microcircuits underlying primate cognitive function.